Originally all x-ray shadowgrams of a human patient were received and recorded on x-ray sensitive film, but it is current medical practice to examine a patient by reception of x-rays on two types of x-ray receptors such as an x-ray image intensifier and film, and modern apparatus must provide for exchanging the positions of the film and intensifier on the x-radiation axis from an x-ray tube. This can be done as shown in U.S. Pat. No. 3,281,598 to Hollstein by retracting the image intensifer and swinging a film holder into the radiation axis in front of the intensifier. Alternatively the film holder may be slid longitudinally into the position vacated by retraction of the image intensifier (Vacher, U.S. Pat. No. 3,614,427; Heitman et al, U.S. Pat. No. 4,298,801). Another method is to rotate an assembly supporting both the film holder and intensifier about the intersection of optical axes of both receptors. A more recent system is to rotate both the image intensifier and film holder jointly 90.degree. and then slide the film holder into the radation axis.
But two major problems arise from the prior methods of interchanging receptors when the receptors are mounted on an x-ray stand with arms for rotation of the x-radiation axis about the patient, and when the film is held, not in a single film plate holder, but in film changer or cassettes which must be over twice as long and twice as thick as each film plate.
The first problem is that a film changer is so thick that, if swung directly into the exposure plane or the x-radiation axis, it will intrude into the zone occupied by the patient risking injury to the patient or disturbance if the patient must be moved.
The second problem is present when the x-ray tube and receptors are mounted on opposite arms of a rotating two-armed support, such as a C- or U-shaped frame. Rotation of the receptors about a point other than their common center of gravity will significantly displace their original center of gravity and destroy the balance of the support arm frame.
It is, of course, necessary to interchange the receptors so that their respective image faces, that is the faces on which the desired x-ray image is formed, move to the same image plane on the radiation axis. But x-ray apparatus in which the receptors are rotated about a point at the intersection of their optical axes so as move their image faces to the same position will necessarily shift their common center of gravity which is not on both optical axes.
Substantially true balance of the rotating frame is important both to safety of the patient under examination and in carrying out of medical routines such as catheterization associated with x-ray examination. If power to the motor rotating the frame should fail, imbalance of the frame could rotate it to a position intruding in the patient zone or preventing ready access to the patient or his life support connections. Imbalance would also prevent delicate manual orientation of the radiation axis by the examining physician and staff as the examination procedure progressed. Adjustment of the rotating frame by hand or by low power, slip clutch motors would be possible if the frame were balanced.
Accordingly it is the object of the present invention to provide a way of interchanging two x-ray receptors at an image plane on the radiation axis while maintaining the balance of rotating arms which support the receptors and the x-ray source.